In petrochemical plants and for reaction process systems for organic compounds such as olefin polymerization heat exchange is a unit process essential for separation systems. If fouling occurs in the heat exchanger, the efficiency of heat exchange is reduced, and the pressure drop across the heat exchanger increases. In extreme cases, it is necessary to terminate production to remove the foulant. In any event it is good practice to seek to reduce build-up that reduces heat exchanger efficiency and could at some point “flake off” and contaminate downstream product(s).
U.S. Pat. No. 7,332,070 issued Feb. 19, 2008 to Nishida et al. assigned to Mitsui Chemicals, Inc. teaches reducing fouling in a cooler by adding one or more nonionic surfactants to a solution containing olefin polymers which is cooled to remove heat of reaction. The patent does not disclose or suggest adding surfactant to a solution containing olefin polymers which is to be heated to phase separate solvent from polymer.
The prevention of fouling of heat exchangers in petroleum refining facilities is illustrated by U.S. Pat. No. 4,200,518 A which discloses a method comprising adding 5 to 99 ppm polyalkylene amine to a hydrocarbon stream. However, the use of polyalkylene amine may raise environmental concerns. JP-A No. 2004-43615 discloses a method of removing fouling materials by adding a dialkyl sulfide to raw oil, but its influence on the environment may also be a concern.
Polyolefins such as polyethylene, polypropylene, ethylene-.alpha.-olefin copolymers and propylene-.alpha.-olefin copolymers are produced by a wide variety of processes such as a solution phase polymerization and gas-phase polymerization. Gas-phase polymerization process results in particulate (granular) (co)polymers, and unlike the solution phase polymerization process, does not need steps such as separation of polymer from a solution.
In the solution phase polymerization process and gas-phase polymerization process, polyolefins are produced by (co)polymerizing olefins in the presence of a solid catalyst such as a solid titanium-based Ziegler-Natta catalyst disclosed in, for example, U.S. Pat. No. 4,952,649 and JP-A No. 7-25946 or metallocene catalyst disclosed in JP-A No. 2000-297114. In those processes, however, as the amount of the product is increased, heat of polymerization is usually increased. One method of removing heat in solution phase polymerization is by withdrawing the polymerizing solution once through a pipe etc. outside of the reactor, passing the solution through a heat exchanger to cool it, and returning it to the reactor. There is also employed a method wherein a part of a gas composed of hydrocarbons such as unreacted monomers (also referred to as “hydrocarbon-containing gas”) is withdrawn continuously from a gaseous phase during liquid phase polymerization or from the top of a reactor during gas-phase polymerization, then the hydrocarbon-containing gas is cooled in a heat exchanger to remove heat of polymerization, and the gas (and a partially liquefied gas) is returned as polymerizable monomers to the polymerization reactor. However, fouling in the heat exchanger for polymer recovery is a problem.
In short residence time solution processes (e.g. the residence time of the solution in the reactor is less than about 15 minutes, preferably less than about 10 minutes), the exothermic heat of reaction may be balanced somewhat by the temperature of the reactants being fed to the reactor. The resulting polymer solution leaving the last reactor in a reactor train may be at a temperature from about 150° C. to 225° C. However, in some instances it is desirable to heat the resulting solution prior to polymer recovery by devolatilizing/flashing the solvent. The step of heating the solution is typically conducted by passing the process stream through a heat exchanger. This may result in the above noted drawbacks. Additionally, as the heat exchanger is hot, any buildup of foulant in the heat exchanger may also result in charring of the material which could potentially flake off causing “black specks” in the resultant polymer product.
Canadian Patent application 2598957, in the name of Cheluget et al, assigned to NOVA Chemicals, teaches adding a surface active agent selected from the group consisting of carboxylate, sulfate, phosphate, phosphonate, and sulfonate compounds comprising a branched or un-branched, saturated or unsaturated alkyl group comprising 6 to 30 carbon atoms, and mixtures thereof to a solution of polyolefins prior to subjecting the solution to flashing (devolatilization). The patent does not disclose or suggest the agents of the present invention.
Applicants have found a dearth of art in the field of reducing fouling of heaters for increasing the temperature of ex reactor solutions of polyethylene to assist in polymer recovery.
The present invention seeks to provide a process for reducing fouling in heat exchangers for heating a solution of polymer in a solvent.